The present invention relates to a polarization-independent optical filter module with isolator function and to an optical amplifier using the same.
In large capacity optical communications systems using optical fibers, optical amplifiers that directly amplify optical signals, without converting optical signals gradually attenuated during transmission into electric signals, are used. FIG. 6 shows the configuration of the optical fiber amplifier disclosed in the book, "optical amplifier and its applications" item 111, written by Ishio et al., OHM Co., (1992). Because the light output from such an optical fiber amplifier contains noise due to natural radiation rays or pumped laser light components, the optical filter that can suppress light except signal light is needed. Moreover, an isolator is needed to prevent the oscillation in the optical fiber due to the return light. FIG. 7 illustrates the configuration of a conventional filter module.
In order to eliminate the polarization-dependence in the conventional optical filter module, an incident light to the optical filter is maintained in a single polarization mode using a polarization splitting prism and a half-wave plate. The optical filter is assembled aslant to the incident light to adjust the transparent center wavelength. However, any polarization dependence is not occur. Such an optical filter is disclosed in JP-A 358115/1992. FIG. 8 shows the configuration of a conventional isolator disclosed in the book, "optical amplifier and its applications" item 153, written by Ishio et al., OHM Co., (1992). The conventional isolator operates in such a manner that the light propagating in the reverse direction is not coupled to the forward incident fiber using a doubly-refracting prism, a polarized-wave rotor, and a Faraday rotator, in spite of its polarized light mode.
The optical fiber amplifier is a device that injects a pumped laser beam into an optical fiber in which a rare earth element such as erbium is doped and amplifies an original optical signal by guiding the optical signal to be amplified. When the pumped laser light source receives return light, noises occur in the optical fiber amplifier because of disturbance of oscillation inside the laser light source. The noises are amplified in a rare-earth-element-doped fiber and interfere with the signal light. Hence in the optical fiber transmission system, an optical isolator that blocks the light propagating in reverse direction as well as an optical filter that removes noises due to pumped light and natural radiation rays are essential. However, since the conventional optical fiber module does not have the isolation function, the optical filter and the optical isolator must be discretely configured. For that reason, the connecting and adjusting works become complicated. The whole system is large sized. Moreover, the conventional optical filter module has the problem in that polarization dispersion occurs because the optical path of the P-polarized light differs from that of the S-polarized light.